Defensive capabilities
There is a thematic aspect to the tank stances too i.e. I'm in tank mode now or I'm in power mode. But that hasn't been the case for a while and seems silly when you've already picked the job that is by its nature a tank or that fact that they're just turn on or off buttons.

Tank stances from a defensive standpoint are a resource intensive skill with quite significant opportunity costs and marginal benefits. Also the differences between the tanks related to this skill are uninteresting and primarily executional.

I don't think there is anything wrong with having a defensive skills tied to resources (TBN, shelltron, IB). I just think it would be better to replacing the tank stance with more unique, defensive skills.
For example, you could still activate a skill that grants defensive bonuses but it runs out and you use skills and your rotation to maintain it (like dragoon) or, like storms eye, a skill could trigger a defensive buff that you could choose to maintain or play with your rotation to have it on when you need it etc